The piles used for jacket type foundation for an offshore wind turbine are subjected to highly cyclic tension and compressive loading. The pile capacity under cyclic axial loading decreases with number of loading cycles due to the reduction of pile shaft resistance. A numerical simulation scheme is presented, which allows the calculation of pile capacity degradation (CDM) due to cyclic loading for driven steel piles. The volume compaction of soil near pile surface during the cyclic loading is determined from cyclic simple shear test results and is then applied to the pile-soil system.
From the limited number of tests available, interaction diagrams have been developed, which give the number of load cycles leading to failure dependent on the mean load and the amplitude of the cyclic load portion, both related to the static pile capacity. However, such diagrams cannot account for different soil conditions or pile geometry and pile stiffness. The calculation results for piles with three different embedded lengths in sandy soil under cyclic loading in tension are presented and compared with the existing interaction diagrams. Finally, recommendations regarding necessary further investigations and improvements of the method are given.
Jacket type foundation of offshore wind turbines are supported by steel pipe piles located at the edges of the foot print area (Fig.1). These piles are often subjected to intensive cyclic axial loads due to wind and waves. A loading time series applied on the pile (pile 4 in Fig. 1) of a reference wind turbine is presented in Fig. 2. Under cyclic axial load, the pile capacity decreases with increasing number of load cycles. This occurs mainly due to a decrease of ultimate skin friction, whereas the tip resistance is believed to remain almost unchanged.
Although numerous studies have been carried out regarding pile capacity under cyclic loading, no approved calculation approach yet exists to determine the decrease of capacity dependent on cyclic load magnitude and number of load cycles. At the institute of the first three authors, a new simulation scheme was developed, which allows the calculation of pile capacity degradation (CDM) due to cyclic loading for driven steel piles [Abdel-Rahman & Achmus (2011a & 2011b), Abdel-Rahman et al.(2014a & 2014b), Achmus et al. (2015)]. Herein, the system behavior under the first loading and unloading step is simulated by a finite element calculation. From the resulting shear strain amplitude in each soil element, the volume compaction to be expected under the considered number of load cycles is predicted by an approach derived from cyclic simple shear tests conducted with sandy soil by Silver & Seed (1971). This volume compaction is applied to the elements in the finite element model and results in a reduction of the normal stresses acting on the pile. In a subsequent loading stage, the (reduced) post-cyclic capacity of the pile is determined.
